One of the testers in the Sun Yang doping case has said he was not a trained doping-control assistant (DCA) but a construction worker.

Sun is accused of not cooperating with three drug testers at his home on Jan 3, with the three-time Olympic champion declaring his innocence at a Court of Arbitration for Sport (CAS) hearing in Montreux, Switzerland, last Friday.

The World Anti-Doping Agency (WADA) has appealed against an initial ruling by swimming's governing body FINA that cleared Sun of any violation.

The tester, who spoke to Xinhua on condition of anonymity, revealed he had provided his testimony in a written letter in Chinese to CAS and WADA several days before the Montreux hearing.

"I am a builder and I am always busy at work, day and night. No one ever trained me in doping tests, and it is unnecessary for me to undertake such training," he said.

"I agreed to testify via a video conference call before the public hearing as they requested. I was ready, but no one has ever contacted me about this."

In an interview with Xinhua in January, the tester said he was providing temporary help for WADA contractor IDTM at the request of his former middle-school classmate, who is a doping control officer (DCO), on the night in question (Sept 4).

The tester noted that he and the DCO had not seen each other for a long time, having met at a school reunion dinner during the Spring Festival in February last year.

"She called me to pick her up at the railway station and drive her to Sun's home that night. In fact, there was another lady with the DCO at the station. She was the BCA(blood collection assistant)," he said.

"Before we entered the test room, the DCO asked me to escort Sun into the bathroom. As I understood it, she was asking me to watch Sun Yang urinate. Because both of them were women, I agreed.

"Sun is a big star in China, so outside the room, I took a couple of pictures with my cellphone.

"When I tried to take pictures of him again when we were sitting in the room, Sun told me not to do so.

"Then he asked every one of us to identify ourselves. I showed my ID card. Sun pointed out that I was not an accredited tester and should not stay in the test room."

During the 11-hour CAS hearing, Sun said that he stopped cooperating with the process because of the tester's unprofessional behavior. Sun later found out that the three IDTM testers lacked sufficient authorization and credentials.

The "temporary" tester said he left the test room and waited outside, adding that the DCO came out to him several times with an iPad and showed him some information on the screen, written in English.

"I did not know English and did not understand what she was talking about. Then I gave the iPad back to her. I didn't know what happened in the test room," he said.

"I knew nothing about the doping test and nothing about my role that night. I just came to help my middleschool classmate at her request. I am a builder."

None of the testers appeared at the CAS hearing, prompting Sun to ask the question: "Do you have the guts to show up and tell the truth here in front of the public?"

A CAS verdict on the case is expected in the coming months.